The mechanisms underlying brain plasticity remain
unknown. We studied for the first time the effects of
working memory training on plasticity in multiple MRI
modalities of white matter microstructure. Demanding
training led to increased axonal density and undemanding
training to decreased axonal density in right
parieto-frontal fibers. Training-induced cognitive
improvements were related to increases in myelin water
fraction. These results are the first demonstrations of
opposing changes in measures of axonal density and
myelin after varying training demands, suggesting that
the intensity and nature of daily activities may
critically shape an individuals cognitive capacities
and underpinning brain infrastructure.

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